A considerable range of fluids are used in the creation and operation of subterranean boreholes. These fluids may contain suspended solids for a number of purposes. Included within this broad category are drilling fluids which may contain suspended solids. One possibility is that a drilling fluid contains solid particles specifically intended to block fractures in formation rock and mitigate so-called lost circulation.
Lost circulation, which is the loss of drilling fluid into downhole earth formations, can occur naturally in formations that are fractured, porous, or highly permeable. Lost circulation may also result from induced pressure during drilling Lost circulation may also be the result of drilling-induced fractures. For example, when the pore pressure (the pressure in the formation pore space provided by the formation fluids) exceeds the pressure in the open borehole, the formation fluids tend to flow from the formation into the open borehole. Therefore, the pressure in the open borehole is typically maintained at a higher pressure than the pore pressure. However, if the hydrostatic pressure exerted by the fluid in the borehole exceeds the fracture resistance of the formation, the formation is likely to fracture and thus drilling fluid losses may occur. Moreover, the loss of borehole fluid may cause the hydrostatic pressure in the borehole to decrease, which may in turn also allow formation fluids to enter the borehole. The formation fracture pressure typically defines an upper limit for allowable borehole pressure in an open borehole while the pore pressure defines a lower limit. Therefore, a major constraint on well design and selection of drilling fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients though the depth of the well.
Several remedies aiming to mitigate lost circulation are available. These include the addition of particulate solids to drilling fluids, so that the particles can enter the opening into a fracture and plug the fracture or bridge the opening to seal the fracture. Documents which discuss such “lost circulation materials” include U.S. Pat. No. 8,401,795 and Society of Petroleum Engineers papers SPE 58793, SPE 153154 and SPE 164748.
One proposal to use particles of organic polymer as lost circulation material is U.S. Pat. No. 7,284,611 which mentions ground thermoset polymer laminate. Particle shape is not mentioned. One supplier of such material refers to it as flakes. This document also mentions an elastomer: again shape is not mentioned. U.S. Pat. No. 7,799,743 mentions granules of polypropylene, which is a thermoplastic polymer and requires particles to have an average resiliency of at least 10% rebound after compression of a quantity of articles by a pressure of 0.4 MPa. The shape of the particles is not mentioned.